A crystallized glass is a material exhibiting unique characteristics owing to various crystals precipitating in the glass. For example, an LAS-type crystallized glass formed by precipitating crystals of a β-quartz solid solution, β-spodumene, or the like in the glass exhibits extremely low expansion or minus expansion, and hence exhibits high mechanical strength and high thermal shock resistance compared with ordinary glasses. Thus, the LAS-type crystallized glass is used for a front glass for a kerosine stove, a wood stove, or the like, a substrate for a high-tech product such as a substrate for a color filter or an image sensor, a setter for firing an electronic part, a tray for a microwave oven, a top plate for induction heating cooking, a window glass for a fire protection door, or the like. For example, Patent Documents 1 to 3 disclose a transparent LAS-type crystallized glass formed by precipitating a metastable β-quartz solid solution (Li2O.Al2O3.nSiO2 [provided that n≧2]) as a main crystal and a white opaque LAS-type crystallized glass formed by precipitating a stable β-spodumene solid solution (Li2O.Al2O3.nSiO2 [provided that n≧4]) obtained by further subjecting a β-quartz solid solution to higher temperature treatment.
By the way, a crystallizable glass, which is mother glass of those crystallized glasses, is generally formed into a plate shape by a roll-out process involving sandwiching a molten glass directly with two refractory rollers and drawing the molten glass. However, the surfaces of the molten glass are directly in contact with the surfaces of the refractory rollers, and hence the roll-out process is apt to cause problems such as imprint of the surface shapes of the refractory rollers and swelling on the surfaces of the glass. Further, the molten glass is forcibly formed into a plate shape by the refractory rollers while being cooled, and hence unevenness is apt to occur. As a result, obtaining a plate glass having a uniform quality is difficult. Thus, a plate glass formed by the roll-out process has involved a problem that more time and more cost are required for its production because the surfaces of the glass need to be subjected to machine polishing to obtain smooth and flat surfaces.
Note that in the roll-out process, the width of a glass plate is restricted depending on the length of the refractory roller, and it is difficult to control the molten glass so that the molten glass extends uniformly in the longitudinal direction of the refractory roller, resulting in easy reduction in quality. Thus, producing a larger glass plate by the roll-out process is difficult.
On the other hand, a float process (float forming process) has been conventionally proposed as another forming process. The float process is good in production efficiency because glass can be formed into a large plate shape, and can provide glass having a high surface quality. The float process is a process involving feeding a molten glass onto a bath of a molten metal bath such as a molten metal tin bath, to thereby form the molten glass into a plate shape. To be specific, a molten glass is fed onto a molten metal bath in a float forming chamber in which a reducing atmosphere is maintained, thereby producing a plate-shaped glass (glass ribbon) having an equilibrium thickness, and the glass ribbon is then drawn on the molten metal bath so as to have a predetermined thickness, to thereby form a plate glass. The float process has been widely adopted as a process of continuously producing large quantities of plate glass products that require a high surface quality (see, for example, Patent Document 4 or 5).